[MacPorts] #64626: gtk-doc: add python310 variant

[MacPorts]

#64626: gtk-doc: add python310 variant
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 Reporter:  nerdling  |      Owner:  mascguy
     Type:  defect    |     Status:  assigned
 Priority:  Normal    |  Milestone:
Component:  ports     |    Version:
 Keywords:            |       Port:
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 gtk-doc should add a python310 variant, the current version of python.

 Also consider dropping python36 variant which is a python version that's
 no longer actively supported upstream.

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Ticket URL: <https://trac.macports.org/ticket/64626>
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